The use of optical fibers as components of chemical sensors for in-situ monitoring of different chemical species is a recent development. For example, various researchers have successfully demonstrated the use of porous fiber sensors for the measurement of humidity, pH, ammonia, and carbon monoxide. In this regard, attention is directed to the works of Shahriari et al referenced in the First International Symposium on Field Screening Methods for Hazardous Waste Site Investigations, Las Vegas, Nev. (1988), Proceedings of the International Society for Optical Engineering, Vol. 838 Fiber Optic and Laser Sensors V, page 348, (August 1987), Optics Letters, Vol, 13, page 407 (May 1988), and Zhou et al (International Journal of Optoelectronics, 1989, Vol. 4, page 415).
The general approach taken with respect to these sensors involves the interaction of light which propagates through the fiber interacting with a reagent that, in turn, selectively interacts with the environment to be sensed. The typical optical properties monitored include evanescant absorption and fluorescence, and chemiluminescence. The reagents are normally immobilized into a membrane or porous polymer matrix and then coated either on the tip or side of the fiber.
One of the problems encountered with fiber optic chemical sensors based on evanescent absorption has been identified by researchers as low sensitivity attributable to the limited depth of penetration of evanescent light into the reagent cladding. These researchers include Giuliani et al (Optical Letters, Vol, 8, page 54, 1983), Russell et al (Anal. Chem. Actal., Vol. 170, page 209, 1985), Ballantine et al (Analytical Chemistry Vol 58 page 2883, 1986), and Zhu et al (Journal of Electrochemical Society, 1987, Vol. 136 (2); page 567).
As part of the effort to overcome this prior art problem, a high sensitivity chemical sensor employing porous glass fibers was designed and developed. The theory underlying this effort was that in a typical evanescent fiber optic sensor, the sensitivity is limited both by the depth of penetration of evanescent light into the reagent coated on the fiber core, and the number of internal reflections, whereas in a porous fiber, the analyte, i.e., the chemical species to be sensed, would penetrate into the pores and interact with the reagent which would have been cast previously into the pores. Since the porous fiber would have a large surface area, it is theorized that the absorption would be enhanced dramatically, leading to an optrode with high sensitivity. Another advantage theorized with a porous glass fiber would be the small sensing region (about 0.5 cm in length and 250 microns in diameter) which would be an integral part of the fiber waveguide. Theoretically, this latter feature would be expected to eliminate the complications associated with the physical and optical coupling of the sensor probe to the fiber. In addition, multiple fiber sensors would be deployed from a single analytical unit and would be expected to be less expensive than conventional sensors based on materials cost and ease of fabrication.